Method to increase the compressive stress of PECVD silicon nitride films

ABSTRACT

Compressive stress in a film of a semiconductor device may be controlled utilizing one or more techniques, employed alone or in combination. A first set of embodiments increase silicon nitride compressive stress by adding hydrogen to the deposition chemistry, and reduce defects in a device fabricated with a high compressive stress silicon nitride film formed in the presence of hydrogen gas. A silicon nitride film may comprise an initiation layer formed in the absence of a hydrogen gas flow, underlying a high stress nitride layer formed in the presence of a hydrogen gas flow. A silicon nitride film formed in accordance with an embodiment of the present invention may exhibit a compressive stress of 2.8 GPa or higher.

CROSS-REFERENCE TO RELATED APPLICATION

This instant nonprovisional application claims priority to commonlyassigned U.S. patent application Ser. No. 60/685,365, filed on May 26,2005 and U.S. patent application Ser. No. 60/701,854, filed on Jul. 21,2005, the entire disclosure of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

In the processing of a substrate to fabricate circuits and displays, thesubstrate is typically exposed to an energized process gas capable ofdepositing or etching material on the substrate. In chemical vapordeposition (CVD) processes, process gas energized by a high frequencyvoltage or microwave energy is used to deposit material on thesubstrate, which may be a layer, a filling of contact holes, or otherselective deposition structures. The deposited layer can be etched orotherwise processed to form active and passive devices on the substrate,such as for example, metal-oxide-semiconductor field effect transistors(MOSFETs) and other devices. A MOSFET typically has a source region, adrain region, and a channel region between the source and drain. In theMOSFET device, a gate electrode is formed above and separated from thechannel by a gate dielectric to control conduction between the sourceand drain.

The performance of such devices can be improved by, for example,reducing supply voltage, gate dielectric thickness, or channel length.However, such conventional methods face mounting problems as the sizeand spacing of the devices become ever smaller. For example, at verysmall channel lengths, the advantages of reducing channel length toincrease the number of transistors per unit area and saturation currentare offset by undesirable carrier velocity saturation effects. Similarbenefits which are obtained from reducing gate dielectric thickness,such as decreased gate delay, are limited in small devices by increasedgate leakage current and charge tunneling through the dielectric whichcan damage the transistor over time. Reducing supply voltage allowslower operating power levels but such reductions are also limited by thethreshold voltage of the transistor.

In a relatively newly developed method of enhancing transistorperformance, the atomic lattice of a deposited material is stressed toimprove the electrical properties of the material itself, or ofunderlying or overlying material that is strained by the force appliedby a stressed deposited material. Lattice strain can increase thecarrier mobility of semiconductors, such as silicon, thereby increasingthe saturation current of the doped silicon transistors to therebyimprove their performance. For example, localized lattice strain can beinduced in the channel region of the transistor by the deposition ofcomponent materials of the transistor which have internal compressive ortensile stresses. For example, silicon nitride materials used as etchstop materials and spacers for the silicide materials of a gateelectrode can be deposited as stressed materials which induce a strainin the channel region of a transistor. The type of stress desirable inthe deposited material depends upon the nature of the material beingstressed. For example, in CMOS device fabrication, negative-channel(NMOS) doped regions are covered with a tensile stressed material havingpositive tensile stress; whereas positive channel MOS (PMOS) dopedregions are covered with a compressive stressed material having negativestress values.

Thus, it is desirable to form stressed materials that have predeterminedtypes of stresses, such as tensile or compressive stresses. It isfurther desirable to control the level of stress generated in thedeposited material. It is also desirable to deposit such stressedmaterials to generate uniform localized stresses or strains in thesubstrate. It is also desirable to have a process that can form stressedmaterials over active or passive devices on the substrate withoutdamaging the devices. It is still further desirable that the depositedfilms be highly conformal to underlying topography.

BRIEF SUMMARY OF THE INVENTION

A plurality of techniques may be employed alone or in combination, toform stressed films having desirable properties. Compressive stress in afilm of a semiconductor device may be controlled utilizing one or moretechniques, employed alone or in combination. A first set of embodimentsincrease silicon nitride compressive stress by adding hydrogen to thedeposition chemistry, and reduce defects in a device fabricated with ahigh compressive stress silicon nitride film formed in the presence ofhydrogen gas. A silicon nitride film may comprise an initiation layerformed in the absence of a hydrogen gas flow, underlying a high stressnitride layer formed in the presence of a hydrogen gas flow. A siliconnitride film formed in accordance with an embodiment of the presentinvention may exhibit a compressive stress of 2.8 GPa or higher.

An embodiment of a method in accordance with the present invention forforming silicon nitride, comprises, disposing a workpiece including asurface in a processing chamber, exposing the surface to a plasma in theabsence of deposition conditions to remove contamination, and depositinga silicon nitride layer over the plasma-treated surface in the presenceof a hydrogen gas flow.

An alternative embodiment of a method in accordance with the presentinvention for forming silicon nitride, comprises, disposing a workpieceincluding a surface in a processing chamber, forming a buffer layer overthe surface, and depositing a silicon nitride layer over the bufferlayer in the presence of a hydrogen gas flow.

Another alternative embodiment of a method in accordance with thepresent invention for forming silicon nitride, comprises, disposing aworkpiece including a surface in a processing chamber, and depositing onthe surface a film stack comprising a silicon nitride initiation layerformed in the absence of the hydrogen gas flow, followed by a highcompressive stress silicon nitride layer formed in the presence of thehydrogen gas flow.

An embodiment of a method in accordance with the present invention forpreventing defects in a device, comprises, treating a surface with aplasma, and depositing a silicon nitride layer on the treated surface inthe presence of a flow of hydrogen gas.

An alternative embodiment of a method in accordance with the presentinvention for preventing defects in a device, comprises, depositing abuffer layer on a surface, and depositing a silicon nitride layer on thebuffer layer in the presence of a flow of hydrogen gas.

Another alternative embodiment of a method in accordance with thepresent invention for preventing defects in a device, comprises,depositing a silicon nitride initiation layer on a surface in theabsence of a flow of hydrogen gas, and depositing a high compressivestress silicon nitride layer on the initiation layer in a presence of aflow of hydrogen gas.

An embodiment of a film stack in accordance with the present inventioncomprises an oxide buffer layer, and an overlying silicon nitride layerdeposited in the presence of hydrogen gas and exhibiting a highcompressive stress.

Another embodiment of a film stack in accordance with the presentinvention comprises a silicon nitride initiation layer interposedbetween a oxide buffer layer and a high compressive stress siliconnitride layer, the initiation layer exhibiting a lower compressivestress than the overlying silicon nitride layer.

A further understanding of the objects and advantages of the presentinvention can be made by way of reference to the ensuing detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross-sectional view of a substrate showing apartial view of a transistor structure with an overlying depositedtensile stressed silicon nitride material;

FIG. 2 plots compressive stress and refractive index for CVD SiN filmsformed under a number of different process conditions;

FIG. 3 shows FT-IR spectra for CVD SiN films formed under a number ofdifferent process conditions;

FIG. 4A plots compressive stress and refractive index for CVD SiN filmsdeposited with different silane flow rates;

FIG. 4B plots compressive stress and refractive index for CVD SiN filmsdeposited with different faceplate-to-wafer spacings;

FIG. 4C plots compressive stress and refractive index for CVD SiN filmsdeposited with different hydrogen gas flow rates;

FIG. 4D plots compressive stress and refractive index for CVD SiN filmsdeposited at different applied power levels;

FIG. 5A plots compressive stress and refractive index for CVD SiN filmsdeposited with and without hydrogen gas;

FIG. 5B plots compressive stress and refractive index for CVD SiN filmsdeposited at different powers and at different temperatures;

FIG. 6 is a graph showing measured tensile stresses for increasing powerlevel of the high RF voltage and different nitrogen plasma treatmentprocess cycles;

FIG. 7 is a graph showing the tensile stress values and refractiveindices obtained for layers deposited under different deposition andnitrogen plasma treatment process cycles;

FIG. 8 is a graph showing the change in tensile stress values ofdeposited materials with N₂ plasma treatment time;

FIG. 9 is a graph showing the effect of N₂ plasma treatment time on thetensile stress value for processes having different purge and pumpcycles;

FIG. 10 plots film tensile film stress versus throughput for CVD SiNfilms formed with different dep/treat cycles;

FIG. 11A shows FT-IR spectra for a CVD SiN film resulting fromdeposition at 400° C. under the various cycle and times shown in TableIV.

FIG. 11B plots the ratio of N—H:Si—N bonds of the CVD SiN filmsresulting from deposition under the various cycle conditions shown inTable IV.

FIGS. 12A-D plotting film stress of a CVD SiN film formed at 450° C.under a variety of different process conditions.

FIG. 13A charts tensile stress and plots reduction in hydrogen contentfor CVD SiN films exposed to post-deposition treatment with plasmaformed from gases containing different levels of Argon, at 400° C.;

FIG. 13B plots tensile stress, H content, and FT-IR spectrum peak areafor N—H and Si—H bonds, for the CVD SiN films deposited in FIG. 13A;

FIG. 13C charts tensile stress and plots reduction in hydrogen contentfor CVD SiN films exposed to post-deposition treatment with an Ar plasmaat different power levels;

FIG. 13D plots tensile stress, H content, and FT-IR spectrum peak areafor N—H and Si—H bonds, for the CVD SiN films deposited in FIG. 13C;

FIG. 13E charts tensile stress and plots reduction in hydrogen contentfor CVD SiN films exposed to post-deposition treatment with plasmaformed from gases containing different levels of Argon, at 550° C.;

FIG. 13F plots tensile stress, H content, and FT-IR spectrum peak areafor N—H and Si—H bonds, for the CVD SiN films deposited in FIG. 13F;

FIG. 14 plots film thickness versus number of dep/treatment cycles, forCVD SiN films deposited with and without Ar plasma cleaning between eachcycle;

FIG. 15 plots film thickness versus number of dep/treatment cycles for,CVD SiN films deposited without Ar plasma cleaning between each cycle;

FIG. 16 plots film thickness versus number of dep/treatment cycles, forCVD SiN films formed under a number of conditions between successivecycles;

FIG. 17 plots films thickness versus number of dep/treatment cycles, forCVD SiN films formed under a number of conditions between successivecycles;

FIG. 18 is a schematic view of an exposure chamber suitable for exposinga silicon nitride material to a suitable energy beam source;

FIG. 19 is a bar graph showing the change in tensile stress values ofmaterial deposited at different process conditions (A, and B) forincreasing ultraviolet radiation exposure time;

FIG. 20 is a graph showing a Fourier Transformed Infrared (FT-IR)spectrum of a stressed silicon nitride material in the as-depositedstate (as dep.-continuous line), and after treatment with ultravioletradiation (treated film—dashed line);

FIGS. 21A to 21E are graphs showing the increase in tensile stress ofdeposited silicon nitride materials with time of ultraviolet radiationexposure, and in FIG. 21A, to both single wavelength (Treatment 1) andbroadband (Treatment 2) ultraviolet exposure;

FIG. 22A plots tensile stress and shrinkage versus depositiontemperature for CVD SiN films exposed to post-deposition treatment withUV radiation;

FIG. 22B plots total H content, and the ratio of FT-IR spectrum peakareas of Si—H and N—H bonds, for the CVD SiN films of FIG. 22A;

FIG. 23 shows FT-IR spectra of a CVD SiN film as-deposited, and aftertreatment with UV radiation;

FIG. 24 shows FT-IR spectra of CVD SiN films exposed to post-depositiontreatment plasma as generated from different gas mixtures;

FIGS. 25A-D are cross-sectional electron micrographs showingconformality of CVD SiN films formed under different processingconditions;

FIGS. 26A-B are enlarged cross-sectional electron micrographs showingmorphology of CVD SiN films formed under different processingconditions;

FIG. 27 plots the rate of deposition of material versus exposure dose.

FIG. 28A plots deposition rate versus exposure dose. FIG. 28B shows across-sectional micrograph showing a feature bearing a layer depositedafter a SiH₄ exposure dose of 500 mT*s.

FIGS. 29A-H are cross-sectional electron micrographs showing morphologyof CVD SiN films formed under different processing conditions;

FIG. 30 is a schematic view of an embodiment of a substrate processingchamber that is a PECVD deposition chamber;

FIG. 31 is a simplified cross-sectional view of a conventional higherpressure processing chamber, and a chamber modified in accordance withan embodiment of the instant invention to operate at lower pressures;

FIG. 32 is a perspective view of the modified chamber shown incross-sectional in FIG. 31.

FIG. 33 is a bar graph showing deposited and post annealed particleperformance with the addition of an oxide layer.

FIGS. 34A-C are graphs showing deposited and post annealed particleperformance when an increased initiation layer is used.

FIG. 35 is a bar graph showing deposited and post annealed particleperformance when an oxide layer is used in combination with an increasedinitiation layer.

FIG. 36 is a bar graph showing deposited and post annealed particleperformance when various approaches are used to improve compressivestress reliability.

FIG. 37A is a drawing highlighting the Fresnel Principle.

FIG. 37B is a cross sectional micrograph illustrating the FresnelPrinciple.

FIG. 38A-B are graphs showing the Brewster angle theory.

FIGS. 39A-M illustrate simplified cross-sectional views of anintegration process flow employing stress from a plurality of sources toenhance device performance.

FIG. 40 is graph showing the response of stressed nitride films whenused with Rapid Thermal Processing versus the composition (Si—H/N—H) andtotal hydrogen content of the film.

FIG. 41 is graph showing a higher etch rate on isolated areas for a postdeposition NF₃ etch-back process.

FIG. 42 is an electron micrograph showing profile changes and PatternLoading Effect (PLE) improvement after an NF₃ etch-back process inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A plurality of techniques may be employed alone or in combination, toenhance conformality and stress in a film formed by chemical vapordeposition (CVD). Embodiments in accordance with the present inventionare particularly suited for forming conformal layers exhibiting tensileor compressive stress which impose strain on an underlying siliconlattice.

In one exemplary application, the tensile or compressive stressedsilicon nitride material is formed on a substrate or workpiece 32 in thefabrication of a MOSFET structure 392—which is illustrated in thesimplified cross-sectional diagram of FIG. 1. The relatively highinternal stress of the deposited and treated silicon nitride material 20induces a strain in a channel region 28 of the transistor 24. Theinduced strain improves carrier mobility in the channel region 28 whichimproves transistor performance, such as for example, by increasing thesaturation current of the transistor 24. The silicon nitride material 20can also have other uses within the MOSFET 24, for example, as an etchstop material. The highly stressed silicon nitride material 20 is alsouseful in other structures, such as other transistors including withoutlimitation, bipolar junction transistors, capacitors, sensors, andactuators. The substrate or workpiece can be a silicon wafer or can bemade from other materials such as germanium, silicon germanium, galliumarsenide and combinations thereof. The substrate or workpiece 32 canalso be a dielectric, such as glass, which is used in the fabrication ofdisplays.

The transistor 24 illustrated in FIG. 1 is a negative channel, orn-channel, MOSFET (NMOS) having source and drain regions 36, 40 that areformed by doping the substrate 32 with a Group VA element to form ann-type semiconductor. In the NMOS transistor, the substrate 32 outsideof the source and drain regions 36, 40 is typically doped with a GroupIIIA element to form a p-type semiconductor. For the NMOS channelregions, the overlying stressed silicon nitride material is fabricatedto have a tensile stress.

In another version, the MOSFET transistor 24 comprises a positivechannel or p-channel MOSFET (PMOS), (not shown) which has source anddrain regions that are formed by doping the substrate with a Group IIIAelement to form a p-type semiconductor. In a PMOS transistor, thetransistor 24 may comprise a substrate 32 comprising an n-typesemiconductor or may have a well region (not shown) comprising an n-typesemiconductor formed on a substrate or workpiece 32 comprising a p-typesemiconductor. The PMOS channel regions are covered with a compressivestressed silicon nitride.

In the version shown, the transistor 24 comprises a trench 44 to provideisolation between transistors 24 or groups of transistors 24 on thesubstrate 32, a technique known as shallow trench isolation. The trench44 is typically formed prior to the source and drain regions 36, 40 byan etch process. A trench side wall liner material (not shown) may beformed in the trench 44 by, for example, a rapid thermal oxidation in anoxide/oxinitride atmosphere, which may also round sharp comers on thetrench 44 (and elsewhere). In one version, the trench 44 may be filledwith material 46 having a tensile stress, which can also be used toprovide a tensile stress to the channel region 28. The deposition of thetrench material 46 which may include the use of a High Aspect RatioProcess (HARP), which may include using an O₃/tetraethoxy silane (TEOS)based sub-atmospheric chemical vapor deposition (SACVD) process. Excesstrench material 46 may be removed by, for example, chemical mechanicalpolishing.

The transistor comprises a gate oxide material 48 and a gate electrode52 on top of the channel region 28 between the source and drain regions36, 40. In the version shown, the transistor 24 also comprises silicidematerials 56 on top of the source and drain regions 36, 40 as well asthe gate electrode 52. The silicide materials 56 are highly conductivecompared to the underlying source and drain regions 36, 40 and gateelectrode 52, and facilitate the transfer of electric signals to andfrom the transistor 24 through metal contacts 54. Depending on thematerials and formation processes used, the silicide materials 56 mayalso comprise a tensile stress and produce tensile strain in the channelregion 28. The transistor shown also comprises spacers 60 and oxide-padmaterials 64 which may be located on opposite sidewalls 68 of the gateelectrode 52 to keep the silicide materials 56 separated during asilicidation process to form the silicide materials 56. Duringsilicidation, a continuous metal material (not shown) is deposited overthe oxide-containing source and drain regions 36, 40 and gate electrode52, as well as the nitride containing spacers 60. The metal reacts withthe underlying silicon in the source and drain regions 36, 40 and gateelectrode 52 to form metal-silicon alloy silicide materials, but areless reactive with the nitride materials in spacers 60. Thus, thespacers 60 allow the overlying, unreacted metal to be etched away whilenot affecting the metal alloy in silicide materials 56.

The length of the channel region 28 is shorter than the length of thegate oxide material 48. The length of the channel region 28 measuredbetween the edges of the source region 36 and the drain region 40 may beabout 90 nm or less, for example, from about 90 nm to about 10 nm. Asthe length of channel region 28 gets smaller, implants 72, also known ashalos, may be counter-doped into the channel region 28 to prevent chargecarriers from uncontrollably hopping from the source region 36 to thedrain region 40 and vice versa.

In the version shown in FIG. 1, the silicon nitride material 20 isformed above the silicide materials 56. The silicon nitride material 20typically acts as a contact-etch stop material as well as providingstrain to the channel region 28. The silicon nitride material 20 iscapable of being deposited to have a stress values ranging fromcompressive to tensile stresses. Selection of stress in the siliconnitride material 20 selects the type of strain provided to the channelregion 28 of the transistor 24.

As just described, film stress and conformality are two keycharacteristics of a film that is designed to impose strain on anunderlying silicon lattice. Incorporated by reference herein for allpurposes is U.S. nonprovisional patent application No. 11/055,936, filedFeb. 11, 2005 and entitled “TENSILE AND COMPRESSIVE STRESSED MATERIALSFOR SEMICONDUCTORS”. This previously-filed patent application describesa number of techniques which may be employed to control stress of adeposited film.

The instant provisional application describes additional techniques forcontrolling stress and conformality of a film formed by chemical vapordeposition (CVD). It has been discovered that both types of stress,namely tensile or compressive, and the stress value of the depositedsilicon nitride stressed material can be set in the deposited materialby controlling processing parameters or by treating the depositedmaterial, as described below. The processing parameters are describedseparately or in particular combinations; however, the invention shouldnot be limited to the exemplary separate or combinations describedherein, but may include other separate or combinations of parameters aswould be apparent to one of ordinary skill in the art.

The following sections address controlling over compressive film stress,tensile film stress, and film conformality, respectively.

I. Compressive Stressed Materials

Deposition process and treatment conditions can be tailored to deposit acompressive stressed material on the substrate or to treat a materialduring or after deposition to increase its compressive stress value.Without being limited by the explanation, it has been discovered that asilicon nitride stressed material having higher compressive stressvalues can be obtained by increasing the RF bombardment to achievehigher film density by having more Si—N bonds in the deposited materialand reducing the density of Si—H and N—H bonds. Higher depositiontemperatures and RF power improved the compressive stress levels of thedeposited film. In addition, higher compressive stresses levels wereobtained in the deposited material at higher kinetic energy levels ofplasma species. It is believed that bombardment of energetic plasmaspecies, such as plasma ions and neutrals, generates compressivestresses in the deposited material because film density increases.

The process gas used to deposit compressive stressed silicon nitrideincludes the silicon-containing and nitrogen-containing gases describedbelow in connection with the formation of tensile stressed materials.Also the general deposition process conditions, such as radio frequencytype and power levels, gas flow rates and pressure, substratetemperature and other such process are about the same as those used forthe deposition of tensile stressed materials, unless otherwisespecified.

To deposit a compressive stressed silicon nitride material, the processgas introduced into the chamber comprises a first component thatincludes a silicon-containing gas, a second component that includes anitrogen-containing gas, and a third component containing carbon, boron,or germanium. The silicon-containing compound can be, for example,silane, disilane, trimethylsilyl (TMS), tris(dimethylamino)silane(TDMAS), bis(tertiary-butylamino)silane (BTBAS), dichlorosilane (DCS),and combinations thereof. The carbon-containing compound can be, inaddition to the compounds mentioned above, ethylene (C₂H₂), propylene(C₃H₆), toluene (C₇H₈), and combinations thereof. The boron andgermanium containing compounds can be dibhorane (B₂H₆), boron chlorides(B₂C₁₄), and germane (GeH₄), respectively. For example, a suitablesilane flow rate is from about 10 to about 200 sccm. Thenitrogen-containing gas can be, for example, ammonia, nitrogen, andcombinations thereof. A suitable ammonia flow rate is from about 50 toabout 600 sccm. The process gas can also include a diluent gas that isprovided in a much larger volume than the reactive gas components. Thediluent gas can also serve both as a diluent and at least partially as areactant nitrogen-containing gas, for example, nitrogen in a flow rateof from about 500 to about 20,000 sccm. Other gases that can be includedcan be inert gases, such as for example, helium or argon, in a flow rateof from about 100 to about 5,000 sccm. The process gas may also containadditional gases such as an oxygen-containing gas, for example, oxygen,when depositing silicon oxy-nitride materials. Unless otherwisespecified, in these processes, the electrode power level is typicallymaintained at from about 100 to about 400 Watts; electrode spacing isfrom about 5 mm (200 mils) to about 12 mm (600 mils); process gaspressure is from about 1 Torr to about 4 Torr; and substrate temperatureis from about 300 to about 600° C.

It has been discovered that the introduction of H₂ gas into thedeposition chemistry may substantially increase compressive stress inthe resulting films that are formed. Table I below lists three separateconditions for deposition of silicon nitride films. TABLE I FILM # SiH₄(sccm) NH₃ (sccm) N₂ (L) Ar (L) H₂ (L) 1 60 30 1 3 0 2 60 30 1 3 1 3 6030 0 3 1

FIG. 2 plots film stress and refractive index for SiN films depositedunder the three separate deposition conditions listed above in Table I.FIG. 2 shows the effect of adding H₂ upon the compressive stressexhibited by the resulting film. FIG. 2 shows that the highestcompressive stress is achieved when the SiH₄/NH₃ ratio is optimized forgiven H₂ and Ar flows, with an N₂/Ar/H₂ ratio of 0/3/1.

FIG. 3 plots FT-IR absorption spectra for silicon nitride film #'s 1 and3 listed above in Table I. The FT-IR spectrum for CVD SiN film #3 mayfairly be contrasted with that of CVD SiN film #1. The spectra of FIG. 2indicate that the nitride film #3 deposited in the presence of hydrogengas, exhibits an increase in intensity at about wavenumber 3330 cm⁻¹.This region of the spectrum corresponds to N—H deformationcharacteristic of tensile stress, indicating an increase in compressivestress.

Within given relative ratios of process gases, other parameters can bevaried to further enhance compressive stress. For example, SiN filmswere deposited by CVD at the 0/3/1 N₂/Ar/H₂ flow rate ratio justdescribed, under different flow rates of SiH₄ and NH₃. These experimentsrevealed a center point of maximum compressive stress (G˜−2.8 GPa) witha SiH₄ flow rate of 60 sccm, and a NH₃ flow rate of 150 sccm.

FIGS. 4A-D indicate that the level of compressive stress in thedeposited film can be further enhanced to above about −2.8 GPa byvarying other process parameters. For example, FIG. 4A plots stress andrefractive index for CVD SiN films deposited at three different SiH₄flow rates. FIG. 4A shows that SiH₄ flow rate may be optimized toenhance compressive stress.

FIG. 4B plots stress and refractive index for CVD SiN films deposited atthree different wafer-to-faceplate spacing distances. FIG. 4B also showsthat this spacing difference may be optimized to enhance compressivestress.

FIG. 4C plots stress and refractive index for CVD SiN films deposited atthree different H₂ flow rates. FIG. 4C shows that the H₂ flow rateparameter may be optimized to enhance compressive stress.

FIG. 4D plots stress and refractive index for CVD SiN films deposited atthree different RF powers. FIG. 4D shows that a maximum compressivestress may be achieved by controlling this process parameter.

FIGS. 5A and 5B show that variation of a number of process parameters incombination, may allow for a CVD SiN film to exhibit a compressivestress approaching −3.0 GPa. Specifically, FIG. 5A plots stress andrefractive index of SiN films deposited with and without hydrogen gas,at three different temperatures. FIG. 5A shows that the film depositedat 480° C. with hydrogen gas, exhibited a compressive film stressapproaching −3 GPa. 5B plots stress and refractive index for SiN filmdeposited with H₂ and Ar at 480° C, with a high frequency power ofeither 75W or 100W. FIG. 5B shows that the film deposited with a lowfrequency of 75W achieved a compressive stress of −3 GPa. For thiscombination of gases/pressure/spacing, the optimum power of 75W resultedin a film having the highest compressive stress.

As just described, deposition of silicon nitride in the presence ofhydrogen gas can produce a desirable enhancement in the compressivestress exhibited by the resulting SiN film. However, it is well knownthat hydrogen gas can easily diffuse through dielectric materials.Moreover, the penetration of such hydrogen into semiconducting regionscan degrade reliability of the device, especially at the transistorlevel. This phenomenon is even more pronounced when high stress filmsare used as etch stop layers.

It has also been observed that hydrogen may accumulate at thenitride/device (NiSix) interface, and this accumulated hydrogen can alsocreate physical defects, such as blistering and delamination duringsubsequent processing steps. An analysis of the residue at thedelamination site revealed a presence of Zn and Na, typical metalcontaminants. The probability of such physical defects increases withthe level of compressive stress, and is more pronounced when the nitridefilm is deposited at a lower temperature.

In accordance with various embodiments of the present invention, threeapproaches may be utilized alone or in combination, to eliminate theoccurrence of defects and thus improve the device reliability when highcompressive stress nitride films are formed by deposition in thepresence of hydrogen gas. In accordance with one embodiment, defects maybe reduced by pre-deposition plasma treatment of the surface that is toreceive the high compressive stress silicon nitride. In accordance withanother embodiment, defects may be reduced by forming a buffer layerover the surface that this to receive the high compressive stressnitride layer, prior to deposition of that nitride layer. In accordancewith still a further embodiment of the present invention, defects may bereduced by forming a SiN layer in the absence of hydrogen, prior todeposition of an overlying high stress SiN film in the presence ofhydrogen gas. Each of these approaches is now discussed in turn below.

In accordance with the first embodiment just mentioned, a plasmapre-treatment step prior to deposition of silicon nitride in thepresence of hydrogen can be employed. This pre-deposition plasmatreatment cleans the wafer surface, removing contamination that couldrender the surface susceptible to penetration by hydrogen, such asresidual silane or metallic contaminants such as Zn and Na. The plasmautilized for this pre-treatment step may be formed from a number ofdifferent ambients, including but not limited to N₂O, O₂ andNH₃-containing plasmas which have been successfully used to reduce thenumber of defects of the nitride film post anneal. The pre treatment maybe applied in the same or in a different processing chamber in which theSiN is deposited. The plasma treatment may conclude prior to thesubsequent deposition step, or may be continuous and extend into the SiNdeposition step. The specific parameters of this plasma pre-treatment,such as duration, power, temperature, and ambient, may vary according tothe particular application in order to achieve the desired effects.

In accordance with the second embodiment of the present inventionmentioned above, defects can be reduced and reliability improved, byforming a buffer layer on the surface that is to receive the highcompressive stress nitride. Such a buffer layer, typically comprisingoxide, will then be located at the nitride/NiSix interface. This oxideserves as a buffer layer, blocking hydrogen diffusing through thedeposited SiN film. Atomic hydrogen reaching the oxide buffer willattempt to combine with other hydrogen atoms to form molecular hydrogen,but will be unsuccessful in doing so owing to the strength of the Si—N,Si—H, and N—H bonds. Specifically, hydrogen diffuses by hopping from oneSi—H or N—H bond to another. To migrate out of the nitride layer intothe oxide layer, and Si—O bond needs to break and an Si—N bond willform. This reaction id not energetically favorable, so the hydrogen willremain trapped in the nitride layer. In this manner, the oxide bufferlayer acts as a wall, preventing gas accumulation at the Si/SiNinterface, and decreasing blistering and post anneal delamination.

FIG. 33 compares contamination exhibited by high compressive stresssilicon nitride layers deposited over oxide buffer layers of threedifferent thicknesses. FIG. 33 shows that even the use of a very thinoxide buffer layers results in a post-anneal area count of about 2adders (defects per wafer) or less.

In accordance with the third embodiment of the present inventionmentioned above, defect count may be reduced, and reliability enhanced,by forming an initiation layer prior to introduction of the hydrogen gasutilized to form the high compressive stress nitride layer. As describedabove, the desired high compressive stress characteristic of the nitridelayer derives from the presence of hydrogen gas during deposition. Inaccordance with this third embodiment, the penetration of this hydrogengas may be reduced by performing the initial stages of the deposition inthe absence of hydrogen, such that the resulting silicon nitrideinitiation layer does not exhibit high compressive stress. Once theinitiation layer is formed, hydrogen gas is introduced into thedeposition gas mixture to imbue the overlying silicon nitride with thedesired level of compressive stress.

The role of the initiation layer is to protect the devices from thepotential surges of electrons occurring in the plasma during thedeposition of the high compressive stress. Proper adjustment of theinitiation layer thickness can also allow it to serve as a barrier forhydrogen diffusion. The initiation layer essentially forms a barrierwhich aids in eliminating hydrogen accumulation.

FIG. 34A shows the number of adders observed by a high compressivestress silicon nitride layer is formed at 400° C. and annealed for 5hours at 400° C., over initiation layers of different thicknesses. FIG.34A shows that use of a thicker initiation layer improves particleperformance post-anneal. FIG. 34A also shows that the post-anneal areacount decreases to less than about 3 adders only for initiation layerthicknesses larger than about 90 Å (deposited for 12 sec), as comparedwith initiation layers having a thickness of about 35 Å (deposited for 5sec).

FIG. 34B shows particle performance of high compressive stress films ofvarying thicknesses formed at 480° C., over an initiation layer of thesame thickness (deposited for 10 seconds). FIG. 34B shows thatdeposition of an initiation layer for 10 seconds improved particleperformance for films up to at least 1500Å in thickness.

FIG. 34C plots film stress versus film thickness for high compressivestress films including an initiation layer deposited for 5 seconds or 10seconds. FIG. 34C reveals that increasing the initiation from 5 secondsto 10 seconds does not significantly change the stress for film having athickness of greater than about 350 Å.

Though the three aforementioned approaches for increasing compressivestress reliability have been explained separately, they may also be usedin various combinations with one another. For example, FIG. 35 plotsparticle count for four different high compressive stress siliconnitride film stacks deposited at 480° C. and then annealed for 5 hoursat 400° C. The first and second film stacks include initiation layersdeposited for 5 and 10 seconds, respectively. The third and fourth filmsinclude initiation layers formed over oxide layers deposited for thetimes indicated.

FIG. 35 shows that use of the thicker initiation layer resulted in goodparticle performance in the “as deposited” film. FIG. 35 shows that filmstacks utilizing an oxide buffer layer (having a thickness of 30-50 Å)under the initiation layer, also exhibited improved particleperformance.

FIG. 35 plots particle count and area count for high compressive stresssilicon nitride layers formed under a variety of different conditions.FIG. 35 shows that pre-deposition plasma treatment with ammonia (NH3) isthe most efficient technique for improving resistance to blistering. Theuse of an oxide buffer layers and initiation layers also showed goodresults.

The three embodiments of the present invention just described, can beemployed to solve integration-related issues with other dielectric filmssuch as low-k dielectrics and high tensile stress silicon nitride usinghydrogen or deuterium in the deposition. In accordance with still otherembodiments of the present invention, Deuterium can also be utilizedinstead of hydrogen during deposition, in order to form silicon nitridefilms with compressive stress greater than 3 GPa.

Post Deposition NF₃ Etch-back Process

As described above, silicon nitride dielectric films may be used as abarrier or etch stop layer for various applications. The film thicknessnon-uniformity across the wafer (e.g., bottom vs. top vs. sidewallthickness) can negatively impact the drive current improvement from onedevice to another. PECVD dielectric films may suffer from a highdeposition rate in isolated areas as well as at the poly-gate uppercorner (e.g., bread-loafing). Modulating process parameters may notsignificantly improve step coverage or pattern loading.

In accordance with one embodiment of the present invention, an NF₃etch-back process modulates a PECVD nitride step coverage and patternloading. Diluted NF₃ plasma can be used to etch back the nitride filmafter deposition to modulate the step coverage profile. This etch-backprocess results in a low etch rate and desired etch uniformity. Theetch-back profile may be similar to that of a PECVD deposition profile.In one embodiment, the NF₃ etch-back process is performed in the samechamber as the deposition process and can be run at the end of thedeposition. Alternatively, the etch-back process can be run in adeposition/etch sequence. NF₃ process parameters may be adjusted so thatthe etch profile can be modulated to match the deposition profile.

FIG. 41 illustrates results of a post deposition NF₃ etch-back process.In FIG. 41, a higher etch rate is shown for the isolated areas.Furthermore, a diluted NF₃ etch-back reduces bottom coverage loading byabout 30% without affecting film stress. This can potentially be used tomodulate the step coverage for other PECVD dielectric films.

FIG. 42 shows the profile change and Pattern Loading Effect (PLE)improvement after NF₃ etch-back of the silicon nitride layer labeled M3.M3 illustrates the nitride bread-loafing profile. The M3 profile hasbeen changed after an NF₃ etch-back process. After post deposition NF₃etch-back of the M3 compressive nitride, the PLE is improved.

H. Tensile Stressed Materials

Without being limited by an explanation, it has been discovered that asilicon nitride stressed material having higher tensile stress valuescan be obtained by a number of techniques employed alone or incombination reducing the net hydrogen content, or the amount ofsilicon-hydrogen and nitrogen-hydrogen bonds (Si—H and N—H bondsrespectively) in the deposited silicon nitride material. It is believedthat lowering the hydrogen content in the deposited material, whichresults in a detectably smaller amount of Si—H and N—H bonds in thesilicon nitride material, gives rise to higher tensile stress values inthe deposited material. It has further been discovered that severaldifferent deposition process parameters, treatments of depositedmaterial, or combinations thereof, can be used to achieve lower hydrogencontent in the deposited material, as described herein.

Incorporated by reference herein for all purposes is “Mechanism ofSiN_(x) Deposition from NH₃-SiH₄ Plasma”, Smith et al., J. Electrochem.Soc., Vol. 137, No. 2 (February 1990). This article attributes theformation of tensile stress in a CVD SiN film, to densification of thefilm in a subsurface zone by the elimination of the volatile ammonia(NH₃) species. Specifically, nitrogen radicals in the plasma mayabstract hydrogen to release the ammonia, leaving dangling Si and Nbonds separated by voids. Stretched Si—N bonds then form and aredetectable by FT-IR analysis by a characteristic peak at 840 cm⁻¹.Constrained by the surrounding material, these stretched Si—N bondscannot relax, resulting in tensile stress.

Various techniques may be employed to enhance the level of the tensilestress that is created. As described in detail below, in accordance withone technique, tensile stress may be enhanced by forming the material inmultiple layers in a plurality of successive deposition/treatment(dep/treat) cycles. In accordance with still another embodiment, tensilestress may be enhanced by depositing the material at a lower temperatureprior to subsequent curing by exposure to radiation.

To deposit a tensile stressed silicon nitride material, the process gasintroduced into the chamber may comprise a first component that includesa silicon-containing gas, a second component that includes anitrogen-containing gas, and a third component containing carbon, boron,or phosphorus. The silicon-containing gas can be, for example, silane,disilane, trimethylsilyl (TMS), tris(dimethylamino)silane (TDMAS),bis(tertiary-butylamino)silane (BTBAS), dichlorosilane (DCS), andcombinations thereof. The carbon-containing compound can be, in additionto the gases mentioned above, ethylene (C₂H₄), propylene (C₃H₆), toluene(C₇H₈), and combinations thereof. The boron and phosphorus containingcompounds can be dibhorane (B₂H₆), boron chlorides (B₂C₁₄), andphosphine (PH₃), respectively. For example, a suitable silane flow rateis from about 5 to about 100 sccm. The nitrogen-containing gas can be,for example, ammonia, nitrogen, and combinations thereof. A suitableammonia flow rate is from about 10 to about 200 sccm. The process gascan also include a diluent gas that is provided in a much larger volumethat the reactive gas components. The diluent gas can also serve both asa diluent and at least partially as a reactant nitrogen-containing gas,for example, nitrogen in a flow rate of from about 5000 to about 30,000sccm. The process gas may also contain additional gases such as anoxygen-containing gas, for example, oxygen, when depositing siliconoxy-nitride materials. Unless otherwise specified, in these processes,typical gas pressures are from about 3 to about 10 Torr; substratetemperatures are from about 300 to 600° C.; electrode spacing is fromabout 5 mm (200 mils) to about 12 mm (600 mils); and RF power levels arefrom about 5 to about 100 Watts.

A. Nitrogen Plasma Treatment Cycles

It was further discovered that the stress values of the as-depositedsilicon nitride material could be increased by treating the depositedsilicon nitride film with a nitrogen plasma treatment step (treat). Sucha treatment cycle can be performed by modifying the deposition processto have two process steps. In the first or deposition process step(dep), a process gas comprising a first component comprisingsilicon-containing gas and nitrogen-containing gas, and a secondcomponent comprising a diluent nitrogen gas, is introduced into thechamber and a plasma is formed from the process gas by applying a highor low frequency voltage to the chamber electrodes. In the second ornitrogen plasma treatment cycle, the flow of the first component of theprocess gas which includes the silicon-containing gas and thenitrogen-containing gas is shut off or substantially terminated; whilethe flow of the second component comprising the diluent nitrogen gas isstill left on, and the high or low frequency voltage supplied to theelectrodes to form the plasma is also maintained. These two processcycles are repeated a number of times during deposition of the siliconnitride material.

Again, without being limited by the explanation, it is believed that thenitrogen plasma cycles further reduce the hydrogen content in thedeposited silicon nitride. It is believed that the nitrogen plasma cyclepromotes the formation of silicon-nitrogen bonds in the depositedsilicon nitride material by removing silicon-hydrogen bonds from thedeposited material. However, since the nitrogen plasma treatment canonly affect a thin surface region of the deposited silicon nitride film,a nitrogen treatment cycle is formed after short deposition processcycles in which only a film of silicon nitride is deposited on thesubstrate that is sufficiently thin to allow nitrogen plasma treatmentto penetrate substantially the entire thickness of the deposited film.If the nitrogen plasma treatment was performed after deposition of theentire thickness of the silicon nitride film, only a thin surface regionof the deposited material would be properly treated.

The modified deposition process comprises a sufficient number ofdeposition cycles followed by plasma treatment cycles to achieve thedesired film thickness. For example, a deposition process comprisingtwenty (20) process cycles that each comprises a first deposition cycleand a second nitrogen plasma treatment cycle, deposited a tensilestressed silicon nitride material having a thickness of 500 angstroms.Each deposition cycles was performed for about 2 to about 10 seconds andmore typically about 5 seconds; and each nitrogen plasma treatment cyclewas performed for about 10 to about 30 seconds, and more typically 20seconds. The resultant deposited tensile stressed silicon nitridematerial had a thickness of 500 angstroms, and the tensile stress valueof the deposited material was increased by the nitrogen plasma treatmentto 1.4 GPa. This represented a 10 to 20% improvement over the tensilestress of the as-deposited silicon nitride material, as shown below inTable II. TABLE II Tensile Film Stress (GPa) Temperature 400° C. 430° C.450° C. 480° C. 500° C. Baseline 1.0 1.1 1.2 1.3 1.35 (Single Material)NPT (1) - 20s Treat 1.3 1.35 1.44 1.44 1.43 NPT (2) - 10s Treat 1.3 1.351.4 1.4 1.43NPT = Nitrogen Plasma Treatment

Table II shows the improvement in tensile stress of a deposited siliconnitride material with increased substrate temperature during deposition,and with/without multiple nitrogen plasma treatment cycles. The baseline(single material) silicon nitride film was deposited in a singledeposition process cycle using the process conditions described above,without nitrogen plasma treatment cycles. The baseline film showed anincrease in tensile stress from 1 GPa to about 1.35 GPa as the substratetemperature was increased from 400 to 500° C. The NPT (nitrogen plasmatreatment) films were deposited with multiple deposition and nitrogenplasma process cycles—where NPT (1) corresponds to 20 second nitrogenplasma treatment cycles and NPT (2) corresponds to 10 second nitrogenplasma treatment cycles. It is seen that for both NPT films, the tensilestress increased from the baseline film with the nitrogen plasmatreatment and also increased with substrate temperature.

FIG. 6 shows the effect of increasing power level of the high RF voltageapplied to the electrodes 105, 109, for different nitrogen plasmatreatment process conditions, on the tensile stress values of thedeposited materials. The first process (A) comprised a deposition stagefor 7 seconds, followed by a plasma treatment stage of 40 seconds,repeated for 20 cycles. The second process (B) involved a depositionstage for 5 seconds, followed by plasma treatment for 40 seconds,repeated for 30 cycles. The third process involved plasma stabilizingstage for 4 seconds, deposition for 5 seconds, and plasma treatment for40 seconds, for 30 cycles. The first and third processes resulted in thehighest tensile stress values, when the high radio frequency was set toa power level of a little over 40 Watts, with tensile stress valuesdecreasing on either side of that peak level. The third process steadilydecreased in tensile stress value for increasing power levels from atensile stress value of a little over a 1000 MPa at a power of 0 Wattsto 900 MPa at a power of 100 Watts. Thus a power level of 20 to 60watts, and more preferably 45 watts, was selected for nitrogenplasma/deposition processes.

FIG. 7 shows the tensile stress values and refractive indices obtainedfor layers deposited under different deposition processes and differentnitrogen plasma treatment cycles. The top line indicates the measuredtensile stress values and the bottom line indicated the measuredrefractive indices. The processes included: a deposition only process; aprocess with a 40 second purge to see the effect without RF power, thatis only thermal impact; a process with a 20 second purge then 20 secondplasma step; a process with a 40 second plasma step; a process with a 20second plasma step then 20 second purge; a process with a 3 second fastpurge than 20 second plasma step; a process with a 3 second pump and 20second plasma step, and a process with a 3 second fast purge and 10second plasma step. The layers were formed by a performing a sequence of30 consecutive cycles.

The highest tensile stress values were achieved with the 3 second pump,20 second plasma and 3 second fast purge, 10 second plasma processes.The lowest tensile stress values were measured for the deposition onlyand 10 second purge processes. Generally, the stress value obtainedmaximizes and evens out for plasma treatment durations longer than 10sec; however, the stress values do not saturate for treatment durationsthat were longer than 20 sec when a pump down cycle was added.

Table III below illustrates the exemplary process conditions for eachstep during the cycle shown in FIG. 7. TABLE III SiH₄ NH₃ N₂ Press.Power Duration STEP (sccm) (sccm) (L) (T) (W) (sec) Stabilize 25 50 20 60 4 Deposit 25 50 20 6 45 5 Pump 0 0 0 TFO 0 30 Fast Purge 0 0 20 TFO 030 Purge 0 0 20 6 0  5-40 Treat 0 0 20 6 45 20-40All steps performed at 400° C., with a wafer-to-faceplate spacing of 430milsTFO = throttle valve fully open.

FIG. 8 shows the effect of the duration of N₂ plasma treatment on thetensile stress values of deposited materials. The tensile stress valuesincrease until a treatment duration of about 10 seconds is reached,after which the tensile stress values appears to “saturate” and do notget much larger. The refractive index increases slightly with increasingtreatment time.

FIG. 9 shows the effect of the treatment duration on the tensile stressvalue for processes having a 3 second fast purge and a 3 second pump.The tensile stress values in FIG. 9 do not appear to “saturate” as muchas those in FIG. 8, even for treatment times up to about 20 seconds.

It has been discovered that the implementation of additional steps inthe nitrogen plasma treatment can result in an even greater enhancementof the level of tensile stress in the resulting film. Table IVsummarizes the process sequence for various different cycles of N₂plasma exposure. TABLE IV Step Time Throughput Process (× # cycles)Thickness Stress Twin Tool Sequence (sec) (Å) RI (MPa) (Wafers/hr) Deponly 134 500 1.847 1000 20 (Baseline) Dep/Treat 10/20 525 1.874 1100 10(DP) (×5) Stab/Dep/ 4/5/20 580 1.892 1180 6 Treat (SDT) (×25) Stab/Dep/4/5/3/5/20 510 1.891 1230 4.5 Pump/Purge/ (×30) Treat (SDPPuT)

FIG. 10 plots film stress versus process throughput, for a twin chambertool, for each of the N₂ plasma exposure cycles shown in Table IV. FIG.10 shows that the addition of steps to each cycle reduces processthroughput.

FIG. 11 shows FT-IR spectra for a 2800 Å-thick CVD SiN film resultingfrom deposition at 400° C. under the various cycles and times shown inTable IV. FIG. 11 shows that the N₂ treatment removes hydrogen from bothSi—H and N—H bonds, and the peaks at 2200 cm⁻¹ and 3330 cm⁻¹respectively decrease. An additional indication that N—H is reduced bythe N₂ treatment is the decrease in the peak/shoulder at 1167 cm⁻¹,which corresponds to the Si—NH—Si bond. This peak 1167 cm⁻¹ becomes morepronounced when there are a significant number of N—H bonds in the film.

Without wishing to be limited by any particular theory, it is believedthat N₂ treatment reduces the hydrogen content in the film leading tothe formation of strained Si—N bonds. By introducing additional steps(such as purge and/or pump) after deposition, the effect of the N₂treatment is enhanced because there are no more deposition gases in thechamber. By contrast, where residual SiH₄ and NH₃ remains in the chamberduring treatment, some deposition continues and treatment is not able topenetrate as well into the material already deposited.

FIG. 11A plots the ratio of N—H:Si—N bonds of the CVD SiN filmsresulting 20 from deposition under the various cycle conditions shown inTable IV. FIG. 11A shows that the addition of steps to the N₂ plasmaexposure cycle can reduce N—H content by up to about 40%.

Table V presents stress results for CVD SiN films formed at increasingtemperatures utilizing a Producer® SE tool. TABLE V Tensile Film Stress(GPa) Temperature (° C.) Cycle 400 430 450 Baseline 1.0 1.1 1.2 D/T 1.11.2 — S/D/T 1.2 1. 1.44 S/D/P/Pu/T 1.3 — 1.4

Table V shows that a CVD SiN film having a tensile stress of 1.5 GPa maybe formed within a thermal budget of 450° C., utilizing the modifiedtensile process regime.

FIGS. 12A-D confirm this result, plotting various attributes of a CVDSiN film formed at 450° C. under different process conditions. FIG. 12Aplots film stress versus NH₃ flow and indicates that a tensile stress of1.5 GPa was achieved. FIG. 12B plots film stress versus N₂ flow, andindicates that a tensile stress of 1.5 GPa was achieved with lower N₂flow rates. FIG. 12C plots film stress versus the total SiH₄ and NH₃flow rate, and shows that film stress is not a strong function of thisprocess parameter. FIG. 12D plots film stress versus applied RF power,and shows that a tensile film stress of 1.5 GPa was achieved with alower RF power.

B. Argon (Plasma Treatment)

As described above, exposure of a CVD film to a plasma including anitrogen containing gas may enhance tensile stress of the film. Inaccordance with another embodiment of the present invention, stress of aCVD film may also be enhanced by exposing the film during and/or afterdeposition to a plasma including Argon gas.

FIGS. 13A-F illustrate properties of a CVD SiN film formed by adep/treat cycle under the conditions shown below in Table VI: TABLE VISiH₄ NH₃ N₂ N₂ + Ar RF Power CYCLE STEP (sccm) (sccm) (L) (L) (W)Deposition (Dep) 60 900 1 — 100 Treatment (Treat) 0 0 — 20 variedPressure = 8.5 TorrWafer-to-faceplate spacing = 300 mils

FIGS. 13A-B show the effect of varying the % of Ar gas flowed during thepost deposition treatment, where deposition and treatment are performedat 400° C. FIGS. 13A-B show that the amount of tensile stress resultingin the deposited film directly correlates with reduction in hydrogencontent ([H]) in the resulting film. FIGS. 13A-B also show that theincrease in tensile stress is a function of Argon gas content andsaturates at a % Ar of greater than about 25%.

FIGS. 13C-D show the effect of varying the RF power level applied duringthe post-deposition Ar treatment, where Ar is 25% of the gas flow andboth the deposition and treatment take place at 400° C. FIGS. 13C-D showthat the increase in tensile stress is fairly insensitive to thetreatment RF power.

FIGS. 13E-F show the effect of varying the temperature upon depositedfilms treated with plasma including varying amounts of Argon.Specifically, the dep/treat cycles of FIGS. 13E-F were performed at 550°C. FIGS. 13E-F confirm that the increase is tensile stress of theresulting film directly correlates with reduction in hydrogen content ofthe film. FIGS. 13E-F also indicate that the treatment is less effectivewhen the deposition takes place at a higher temperature (i.e., 550° C.vs. 400° C.). FIGS. 13E-F show the total hydrogen content in the “asdeposited” film is lower when compared to the film deposited at 400° C.(FIGS. 13A-B) which leads to a lower reduction in total hydrogen contentduring plasma treatment.

C. Ultraviolet Radiation Exposure

The tensile stress of an as-deposited silicon nitride material can befurther increased by treating the deposited material with exposure to asuitable energy beam, such as ultraviolet radiation or electron beams.It is believed that ultraviolet and electron beam exposure can be usedto further reduce the hydrogen content in the deposited material. Theenergy beam exposure can be performed within the CVD chamber itself orin a separate chamber. For example, a substrate having the depositedstressed material could be exposed to ultraviolet or electron beamradiation inside the CVD processing chamber. In such an embodiment, theexposure source could be protected from the CVD reaction by a shield orby introducing the exposure source into the chamber subsequent to theflow of process gas. The ultraviolet or electron beams could be appliedto the substrate, in-situ in the CVD deposition chamber during a CVDreaction to deposit the stressed material. In this version, it isbelieved that ultraviolet or e-beam exposure during the depositionreaction would disrupt undesirable bonds as they are formed, therebyenhancing the stress values of the deposited stressed material.

FIG. 18 shows an exemplary embodiment of an exposure chamber 200 whichcan be used to expose a substrate 32 to ultraviolet radiation orelectron beam treatment. In the version shown, the chamber 200 includesa substrate support 104 moveable between a released position distal fromthe exposure source 204, and a lifted position proximate to the source204 to allow adjustment of the spacing therebetween. A substrate support104 supports the substrate 32 in the chamber 200. During insertion andremoval of the substrate 32 from the exposure chamber 200 , thesubstrate support 104 can be moved to a loading position, andthereafter, during exposure of the substrate 32 having the depositedsilicon nitride material to ultraviolet radiation or electron beams, thesupport 104 is raised into the lifted position to maximize exposurelevels. The chamber 200 further comprises a heater 206, such as aresistive element, which can be used to heat the substrate 32 to adesired temperature during exposure of the substrate 32. A gas inlet 208is provided to introduce a gas into the exposure chamber 200 and a gasoutlet 210 is provided to exhaust the gas from the chamber 200.

The exposure chamber 200 further includes an exposure source 204 thatprovides a suitable energy beam, such as ultraviolet radiation orelectron beams. A suitable ultraviolet radiation source can emit asingle ultraviolet wavelength or a broadband of ultraviolet wavelengths.A suitable single wavelength ultraviolet source comprises an excimerultraviolet source that provides a single ultraviolet wavelength of 172nm or 222 nm. A suitable broadband source generates ultravioletradiation having wavelengths of from about 200 to about 400 nm. Suchultraviolet sources can be obtained from Fusion Company, USA or NordsonCompany, USA. The stressed silicon nitride material may be exposed toultraviolet radiation having other wavelengths that are generated bylamps that contain gas that radiates at specific wavelengths whenelectrically stimulated. For example, suitable ultraviolet lamp maycomprise Xe gas, which generates ultraviolet radiation having awavelength of 172 nm. In other versions, the lamp may comprise othergases having different corresponding wavelengths, for example, mercurylamps radiate at a wavelength of 243 nm, deuterium radiates at awavelength of 140 nm, and KrCl₂ radiates at a wavelength of 222 nm.Also, in one version, generation of ultraviolet radiation specificallytailored to modify the stress value in the deposited stressed materialcan be accomplished by introducing a mixture of gases into the lamp,each gas capable of emitting radiation of a characteristic wavelengthupon excitation. By varying the relative concentration of the gases, thewavelength content of the output from the radiation source can beselected to simultaneously expose all of the desired wavelengths, thusminimizing the necessary exposure time. The wavelength and intensity ofthe ultraviolet radiation can be selected to obtain predeterminedtensile stress value in the deposited silicon nitride material.

The CVD deposition chamber 80 and exposure chamber 200 may also beintegrated together on a multi-chamber processing platform (not shown)served by a single robot arm. The exposure source 204 and the support ofthe exposure chamber 200, and the components of the CVD depositionchamber 80 that include the substrate support 104, motor, valves or flowcontrollers, gas delivery system, throttle valve, high frequency powersupply, and heater 206, and the robot arm of the integrated processingsystem, may all be controlled by a system controller over suitablecontrol lines. The system controller relies on feedback from opticalsensors to determine the position of movable mechanical assemblies suchas the throttle valve and substrate support 104 which are moved byappropriate motors under the control of the controller.

For exposure treatment in the described exposure chamber 200, asubstrate having a silicon nitride material according to any of thedeposition processes described or other deposition processes known inthe art, is inserted into the exposure chamber 200 and placed upon thesubstrate support 104 in the lowered position. The substrate support 104is then raised to a lifted position, the optional heater 206 in thesupport powered on, and the exposure source 204 is activated. Duringexposure, a gas may be circulated through the exposure chamber 200, suchas helium, to improve thermal heat transfer rates between the substrateand the support. Other gases may also be used. After a period ofradiation exposure, the exposure source 204 is deactivated and thesubstrate support 104 is lowered back into the released position. Thesubstrate bearing the exposed silicon nitride stressed material is thenremoved from the exposure chamber 200.

FIG. 19 is a bar graph showing the effect of ultraviolet radiationtreatment on the tensile stress values of materials deposited atdifferent process conditions including A: compressive film (45 sccmSiH₄/600 sccm NH₃/2000 sccm He/30W HF/30W LF/2.5T/480 mils/430° C.); andB: tensile film (75 sccm SiH₄/1600 sccm NH₃/5000 sccm N₂50W HF/5 W LF/6T/480 mils/430° C.). Different broadband UV treatment times at 400° C.of 5 minutes and 10 minutes were used. For all deposited films,ultraviolet radiation exposure increased tensile stress values, with thegreatest improvement occurring for the materials having the lowesttensile stress values, namely materials A and B. A and B increased in atensile stress of level from about −1500 MPa to around about −1300 MPa.Materials C and D also increased. Thus, the ultraviolet treatment canincrease the tensile stress value for deposited materials.

It was determined that exposure of the deposited silicon nitridematerial to ultraviolet radiation or electron beams is capable ofreducing the hydrogen content of the deposited material, and therebyincreasing the tensile stress value of the material. It is believed thatexposure to ultraviolet radiation allows replacement of unwantedchemical bonds with more desirable chemical bonds. For example, thewavelength of UV radiation delivered in the exposure may be selected todisrupt unwanted hydrogen bonds, such as the Si—H and N—H bond thatabsorbs this wavelength. The remaining silicon atom then forms a bondwith an available nitrogen atom to form the desired Si—N bonds. Forexample, FIG. 20 shows a Fourier Transformed Infrared spectrum (FT-IR)of a stressed silicon nitride material in the as-deposited state (asdep.—continuous line), and after treatment with ultraviolet radiation(treated film—dashed line). From the FT-IR spectrum, it is seen thatafter treatment with the ultraviolet radiation, the size of both the N—Hstretch peak and the Si—H stretch peak significantly decrease, while thesize of the Si—N stretch peak increases. This demonstrates that afterultraviolet treatment, the resultant silicon nitride material containsfewer N—H and Si—H bonds, and an increased number of Si—N bonds whichare desirable to increase the tensile stress of the deposited material.

FIGS. 21A to 21E show the improvement in tensile stress value of anas-deposited silicon nitride material that is subjected to differentperiods of ultraviolet exposure treatment times. The silicon nitridematerial of FIG. 21A was deposited under the following processconditions 60 sccm flow rate of silane; 900 sccm flow rate of ammonia;10,000 sccm flow rate of nitrogen; 6 Torr process gas pressure;electrode power level of 100 watt; and electrode spacing of 11 mm (430mils). The tensile stress of the deposited silicon nitride film wasmeasured in the as-deposited state to be about 700 MPa. The points label0 to 6 on the x-axis each correspond to different ultraviolet treatmenttime of 0 minutes (as deposited), 10 minutes, 30 minutes, 45 minutes,one hour, two hours, and three hours, respectively. The as-depositedsilicon nitride material of the line labeled with tetrahedrons(treatment 1) was exposed to a broadband ultraviolet radiation source,while the as-deposited silicon nitride material of the line labeled withsquares (treatment 2) was exposed to a single wavelength ultravioletsource at 172 nm. It was determined that the broadband ultravioletradiation source provided increased tensile stress in the depositedmaterial as compared with a single wavelength ultraviolet radiationsource.

Generally, as ultraviolet treatment time increased, the tensile stressof the as-deposited film also increased from the original value of 700MPa to values exceeding about 1.6 GPa. The silicon nitride material ofFIGS. 21B and 21C were deposited under the same conditions as the sampleshown in FIG. 21A, with the following exceptions—the sample of FIG. 21Bwas deposited using 60 sccm flow rate of silane; 600 sccm flow rate ofammonia; and electrode power level of 150 watts; and the sample of FIG.21C was deposited using 60 sccm flow rate of silane; 300 sccm flow rateof ammonia; and an electrode power level of 150 watts. In FIGS. 21B and21C, the as-deposited material was treated only with a broadbandultraviolet radiation, and the treatment times also varied from 0minutes to 3 hours but at different time intervals corresponding to 8 or9 segments, as shown. The best result obtained is shown in FIG. 21C,where the as-deposited silicon nitride material increased in tensilestress after approximately three hours of ultraviolet exposure from 800MPa to 1.8 GPa, which was almost double the original tensile stressvalue.

The material deposited shown in FIG. 21D was deposited using 60 sccmflow rate of silane; 900 sccm flow rate of ammonia; 10,000 sccmnitrogen; electrode power of 100 watt; pressure of 7 Torr; and 11 mmspacing. Line (a) was treated with a Fusion H UV light source whichprovided UV wavelengths of about 200 to 400 nm; and Line (b) was treatedwith an Excimer UV source which provided UV wavelengths of about 172 nm.For both treatments, tensile stresses increased from about 800 MPa (forthe as-deposited silicon nitride) to 1.8 and 1.4 GPa, respectively,after about 50 minutes of ultraviolet exposure material. The cure timecan be significantly reduced by further optimizing the UV lamp toincrease the light intensity reaching the wafer. The sample of FIG. 21Ewas deposited using 60 sccm flow rate of silane; 300 sccm flow rate ofammonia; 10,000 sccm nitrogen; electrode power of 150 watt; pressure of6 Torr; and 11 mm spacing. The deposited material was treated with aFusion H source. As before, the as-deposited silicon nitride materialincreased in tensile stress after approximately 50 minutes of treatmentfrom about 700 MPa to 1.6 GPa.

In the manner just described, tensile stress of a CVD material may beenhanced by post-deposition exposure to ultraviolet radiation. Thisstress enhancement may be accomplished by varying process parameterssuch as UV treatment time, and diluent gas content at the time ofdeposition.

It was also determined that the effect of ultraviolet exposure could beenhanced by optimizing the composition of the “as deposited” film. Ithas further been discovered that the tensile stress increases byincreasing hydrogen content in the “as 10 deposited” film and adjustingthe ratio of Si—H/N—H bonds to about 1:1. The total hydrogen content inthe film can be increased by decreasing the deposition temperature priorto UV exposure. Specifically, reducing the temperature at the time ofdeposition can enhance the tensile stress imparted to a film that issubsequently cured by exposure to UV radiation.

Table VIII lists FT-IR spectral data for two CVD silicon nitride filmsexposed to UV radiation post-deposition. The first CVD nitride film wasdeposited at 400° C., and the second nitride film was deposited at 300°C. TABLE VIII FT-IR SPECTRA DATA OF DEPOSITED FILM AFTER UV CURE (FT-IRdata normalized to 3000 Å) NH Peak SiH Peak SiH Peak DEPOSITION CenterNH Peak Center SiH Peak Center SiN Peak TEMPERATURE (cm⁻¹) Area (cm⁻¹)Area (cm⁻¹) Area 400° C. As Dep 3355 0.033 2159 0.031 842 0.491 UVTreated 3355 0.022 2170 0.024 836 0.497 % Change — 33.8 — 23.5 — 1.1300° C. As Dep 3359 0.042 2146 0.05 847 .43 UV Treated 3360 0.011 21710.01 833 0.5 % Change — 73.8 — 70.8 — 17.0

Table VIII indicates that post-deposition UV treatment reduces thenumbers of both Si—H and N—H bonds, while increasing the network of Si—Nbonds. Without being limited to a particular explanation, Table VIIIlikely indicates that deposition of a lower temperature allows more filmrestructuring during the UV cure step, leading to higher tensile stressin the resulting film.

FIG. 22A plots stress and film shrinkage after UV cure, of a pluralityof CVD nitride films deposited at different temperatures. FIG. 22Aindicates an increase in stress with decreased deposition temperature.FIG. 22A also indicates an increase in shrinkage with decreaseddeposition temperature. The correlation of increased shrinkage withreduced deposition temperature of FIG. 22A confirms the greaterrestructuring undergone by the films initially deposited at lowtemperatures.

FIG. 22B plots total H content ([H]), and SiH/NH peak area ratio; of SiNfilms formed by CVD at different temperatures. FIG. 22B shows thattensile stress of the film increases with hydrogen content at lowerdeposition temperatures.

FIG. 23 plots FT-IR spectra of a CVD nitride film exhibiting asdeposited at 300° C., and then after exposure to UV radiation at 400° C.Table IX lists composition of the CVD SiN film as-deposited, and afterUV curing, as determined by Rutherford Backscattering Spectrometry (RBS)and the Hydrogen-Forward Scattering (HFS) method: TABLE IX RBS/HFS H(%)N(%) Si(%) Si/N As Dep Film 25 43 32 0.74 Post UV Treat Film 16.5 48.535 0.73

FIG. 23 and Table IX indicate that a lower deposition temperatureresults in formation of more SIN bonds after the UV cure, leading tohigher stress in the cured film.

Both the N₂ treatment and UV treatment are based on the same principle.Specifically, Si—H and N—H bonds are broken and hydrogen is removed fromthe film. This hydrogen removal leaves Si and N dangling bonds in thefilm, allowing new Si—N bonds to be formed. Those new Si—N bonds arestrained, because the Si and N atoms are locked in place by the networkand can't more to relieve the strain.

However, the N₂ treatment technique is limited by de penetration depthof the N radicals/ions, and the energy of those N radicals/ions.Increasing the energy may be detrimental, because the N will becomeimplanted in the film, decreasing tensile stress.

By contrast, the UV treatment technique has a bulk effect. The entirefilm can be treated at once and the process is more efficient and canbreak more bonds. Also, because a broadband UV source emittingwavelengths down to 200 nm is being used, the UV energy also favorsre-bonding of the dangling bonds to form the strained Si—N bonds.Specifically, some dangling bonds remain during the formation of allfilms. These dangling bonds have the effect of degrading electricalproperties of the film. These dangling bonds can survive subsequenttreatment, especially if the distance between a Si dangling bond and a Ndangling bond is too large. The UV treatment technique provides thenecessary activation energy to allow the two types (Si and N) ofdangling bonds to form a desired Si—N bond.

As discussed above, UV treatment is one of the techniques used toincrease the tensile stress of the nitride layers. The efficiency of theUV cure is directly correlated with the optical properties of thesilicon nitride layer and the substrate topography.

An increase of UV cure efficiency can improve the stress level and themanufacturability of high stress nitride film. The Fresnel principle asshown in FIG. 34A, illustrates that if the gate to gate spacing is ofthe same order of magnitude as the UV light wavelength, diffraction willoccur leaving some areas un-treated, such as sidewall and bottom cornerof the poly gate. FIG. 34B shows an illuminated region photo with darkbands against the edge followed by light and dark bands. FIG. 34Bconfirms the Fresnel principle by showing plenty of dark areas aroundthe bottom and side walls.

UV cure efficiency can be improved by properly engineering the incidenceof the UV light and the device sidewall profile in order to avoid theFresnel effect and take advantage of the Brewster angle theory.According to this theory, the light absorption is optimal for a criticalangle calculated from the refractive index ratio of the vacuum andnitride. For some SiNx film, this angle is calculated to be between 63and 66 degrees. FIG. 35A illustrates this theory by defining thegeometrical orientation and the p- and s-components of polarization.FIG. 35B shows the reflectance for each component as a function of theangle of incidence, indicating a minimum at Brewster's angle for thep-component. At this Brewster angle, absorption is at a maximum as thereis no diffraction.

Two methods may be utilized in accordance with embodiments of thepresent invention in order to take advantage of the Brewster angletheory and ensure even treatment of different locations on the film bythe UV light. In accordance with a first embodiment, the substrate maybe moved relative to the UV light source, to ensure that light isincident over a variety of angles, including the Brewster angle. Inaccordance with an alternative embodiment, the raised features on thesubstrate may be formed having sidewalls of less than 90 degrees,thereby allowing light incident at the Brewster angle to penetrate reachthe substrate surface.

As mentioned above, the maximum absorption occurs when the angle betweenthe film normal and the direction of UV incidence is between 63 and 66degrees. For blanket wafers or features covering large areas, the filmhas only one orientation relative to the wafer surface and makes a fixedangle with the incident light. Thus in accordance with one embodiment ofthe present invention, the direction of UV incidence can be modified byrotating the source around its axis and/or on a hemisphere above thewafer, or by rotating the substrate relative to the source. Thisrotation ensures that every section of the nitride film on the wafers isexposed to a UV light with a 63-66 degree angle of incidence.

In accordance with an alternative embodiment of the present invention,UV absorption by the nitride film can be enhanced where the devicestructure is modified to offer an angle less than 90 degrees with thewafer surface. On patterned wafers, the film follows the device contourand the angel between the UV incidence and film normal, varying from 0to 180 degrees. Where the features form less than a 90 degree angle, theprobability of satisfying the Brewster angle criterion is increasedwhich leads to an enhancement in UV absorption and a direct increase inthe tensile stress.

The UV source rotation embodiment and the device angle engineeringembodiment may also be used together to enhance efficiency of UV curing.These two embodiments can also be applied for enhancing the post-UV cureproperties of other films such a low-k dielectrics.

In accordance with still other embodiments of the present invention, UVcure efficiency can be enhanced by the addition of porogens. The effectof the UV cure is directly correlated to the UV lamp efficiency and thecuring potential of the deposited film. The curing potential relates tothe change in the film structure dining cure. The structural changeinvolves elimination of hydrogen and reconstruction of the nitrideamorphous network. These structural changes in turn lead to proportionalchanges in film properties, namely, film refractive index and densityincrease, the film shrinks, and residual stress in the film becomes moretensile. To maximize the post-cure tensile stress, the hydrogen contentin the film needs to be maximized while maintaining a balance betweenSiH and NH content in the film. The hydrogen content in the nitridefilms is a strong function of the deposition temperature and is limitedto about 30%.

In accordance with an embodiment of present invention, varioustemperature-labile molecules can be introduced to the depositionchemistry for forming a silicon nitride film, in order to enhance itscuring potential. Such temperature labile molecules are usually of largesize, and are incorporated into the film during deposition withoutbreakage.

After deposition, the molecule can be removed using UV treatment orin-situ plasma treatment. During the post-deposition cure process, thespace in the film previously occupied by the temperature labile moleculewill close, resulting in strained Si—N bonds and increasing tensilestress in the film. The temperature-liable molecules can include but arenot limited to those listed in the following TABLE X: TABLE XTEMPERATURE LABILE MOLECULES (POROGENS) Name Chemical Formulaalpha-terpinene (ATRP) C₁₀H₁₀ Toluene C₇H₈ Limonene C₁₀H₁₆ Pyran C₆H₁₀O₂Vinyl acetate C₄H₆O₂ Cyclo-pentene C₈H₁₄ 1 methyl cyclo-penetne C₆H₁₀ 5vinyl bicyclo hept-2-ene C₉H₁₂ Cyclo-pentene oxide C₅H₈O

III. Strain-inducing Spacer

Still another embodiment in accordance with the present invention offersan integration scheme useful for further enhancing performance of anNMOS device by taking advantage of the change in stress of the nitridelayer induced by the spike annealing process. FIGS. 39A-M illustratesimplified cross-sectional views of the process steps for thisintegration.

As shown in FIG. 39A, the starting point for the process integration isa precursor CMOS structure 3900 comprising PMOS region 3902 separatedfrom adjacent NMOS region 3904 by shallow trench isolation (STI)structure 3906. Gate oxide layer 3908 and overlying gate polysiliconlayer 3910 are formed in a stack over precursor CMOS structure 3900.

FIG. 39B shows the patterning of photoresist mask 3912 to define gap3912 a exposing the gate polysilicon/oxide stack at the location of theNMOS gate. FIG. 39C illustrates the pre-amorphization processesperformed on the polysilicon in the exposed region. Two possible methodsfor such pre-amorphization include 1) the implantation of germanium intothe NMOS poly, or 2) recessing the NMOS poly-silicon gate followed byselective SiGe deposition. With regard to the second alternative, anoxide mask may be used to ensure results.

FIG. 39D shows performance of standard processing steps to form gates3903 and 3905 of the PMOS and NMOS transistor structures, respectively.This conventional processing includes use of disposable (sacrificial)spacers for source/drain implants, followed by the Halo implants.

The imposition of tensile stress enhances the speed of electron flowacross the NMOS channel region. Conversely, the imposition ofcompressive stress enhances the speed of movement of holes in the PMOSchannel region. Accordingly, FIG. 39E shows the deposition of thetensile-stressed nitride film 3930 over the PMOS and NMOS gates prior torapid thermal processing (RTP).

FIGS. 39F-G show removal of the tensile stressed nitride layer over PMOSregions. As shown in FIG. 39F, a mask 3931 is first patterned to exposethe SiN overlying the PMOS region. In FIG. 39G, the exposed SiN isselectively etched utilizing the mask, which is then removed.

FIG. 39H show performance of a RTP spike annealing step, which elevatesthe stress of the conformal nitride film from <1 GPa to about 2 GPa.This RTP spike anneal creates a stress-inducing texture 3932 in thepolysilicon gate. Alternatively, the annealing during this step may takethe form of a dynamic surface anneal used to activate dopants. Theseannealing methods or any other annealing methods may be used torecrystallize the polysilicon of the NMOS gate, thereby increasingnitride stress to 2.0 GPa. The tensile stress imposed by this filmserves to enhance the performance of the NMOS device.

The composition of the silicon nitride layer can be optimized to resultin highest tensile stress of the SiN film after RTP. FIG. 40 plots filmstress versus the RTP temperature spike, for deposited SiN films ofdifferent compositions. FIG. 40 shows the response of PECVD nitridefilms to RTP, vs. the composition (Si—H/N—H) and total hydrogen contentof the film. The stress in the SiN film post RTP is 2 GPa (tensile), andthis stress value can potentially be increased by further optimizing thedeposition chemistry.

FIGS. 39I-L shows the next series of steps in the integration processflow, wherein nitride spacers are formed adjacent to gateoxide/polysilicon stack, to complete formation of the gate structures.Specifically, in FIG. 39I, nitride layer 3934 of neutral or compressivestress is formed over the entire structure. As shown in FIGS. 39J-K,lithography and etching are employed to remove nitride layer 3934 fromthe NMOS region.

FIGS. 39L illustrates formation of the spacer structures 3950 and 3952for the NMOS and PMOS devices, through etching of the tensile stress SiNlayer 3930 and the neutraucompressive SiN layer 3934, respectively.

Finally, FIG. 39M shows the dual stress layer integration. Halo implantsare performed, and conducting contacts such as NiSix are formed,followed by formation of the nitride etch stop layer (ESL). Over theNMOS device, a SiN ESL 3936 exhibiting tensile stress is created. Overthe PMOS device, a SiN ESL 3938 exhibiting compressive stress iscreated.

Once deposited, the silicon nitride etch stop layer may be treated toenhance its tensile stress. For example the deposited etch stop layermay be subjected to in-situ plasma treatment. Alternatively or inconjunction with plasma treatment, the deposited etch stop layer may besubjected to a UV cure with, or without a capping layer to modulate theradiation experienced by the film. Examples of such capping layersinclude but are not limited to amorphous carbon, oxynitride, or othermaterials having extinction coefficients different from the high stressnitride layer.

The integration scheme shown FIGS. 40A-L take advantage of severaldifferent sources of tensile stress in order to improve deviceperformance. First, the tensile stress of the nitride spacer layer isutilized. A second source of stress is that induced in the polysilicongate by the RTP step. A third source of stress is from the nitride etchstop layer formed over the gate.

IV. Enhanced Film Conformality

The above description has focused upon the enhancement of film stress.However, as shown and described above in connection with FIG. 1, anotherimportant property of a film intended to impose strain on a siliconlattice is conformality. Embodiments in accordance with the presentinvention allow the enhancement of conformality of a CVD film, bypermitting deposition and treatment to be performed at low pressures,thereby obviating the need for separate, time-consuming purge stepsbetween film deposition and treatment.

The substrate processing techniques described so far have been performedat pressures of about 1 Torr or greater. As shown and described above,however, a cycle in a process for CVD of a film exhibiting controlledproperties, may involve successive deposition and treatment underdifferent conditions.

Where the processing chamber is operating at pressures of about 1 Torrand above, such changed conditions may generally necessitate pumping orpurging steps in order to achieve optimum results. However, as shown andillustrated above in connection with Table IV and FIG. 10, such anintervening pumping/purging steps can consume significant amounts ofprocessing time, substantially reducing throughput.

Accordingly, embodiments of the present invention also relate to methodsand apparatuses for depositing films by chemical vapor deposition atrelatively low pressures (i.e. between about 20-150 mTorr). The pumpingrequired to maintain the chamber in this low pressure range ensures ashort residence time for gases employed for deposition and treatment,thereby obviating the need for a separate pumping or purging steps.

It has further been discovered that CVD processing at low pressures, andthe concurrent elimination of separate intervening gas pumping/purgingsteps, sufficiently reduces processing time and elevates throughputenough to render commercially practicable the formation of highlyconformal SiN films. In particular, the highly conformal CVD SiN filmsare formed by repeated cycles in which an initial step involving asilicon precursor in the absence of a plasma, results in deposition of ahighly conformal layer of amorphous silicon (a-Si). This deposition stepis followed by a treatment step in which the conformal a-Si film isexposed to a nitrogen-containing plasma. This cyclic processing regimeis rendered commercially practicable by eliminating the need forseparate gas pumping and purging steps intervening between thesuccessive deposition and treatment steps of the cycle.

In accordance with one embodiment of the present invention, a conformalSiN layer may be formed by employing a cyclic deposition process at lowpressure wherein a silane soak deposition step in the absence of plasmais followed by treatment with a plasma formed from N₂ as thenitrogen-containing species. In certain embodiments, the plasma may alsoinclude argon, which may bombard the deposited film and/or assist in thedissociation of N₂, thereby decreasing N—H content in the deposited filmand forming dense N—H bonds.

In accordance with an alternative embodiment of the present invention, aconformal SiN layer may be formed by employing a cyclic depositionprocess at low pressure, wherein a silane soak deposition step in theabsence of plasma is followed by treatment with a plasma formed from N₂and NH₃ as the nitrogen-containing species.

FIG. 24 shows an FT-IR spectra of CVD SiN films formed by a 20 secsilane soak followed by treatment by a 10 sec exposure to a plasmaformed from N₂+Ar, or from N₂+NH₃. FIG. 24 shows that the presence ofargon during treatment may have resulted in a decrease in N—H content,and the formation of dense SiN bonds.

In another experiment, this SiN process regime was performed with, andwithout, a post SiH₄ soak purging step. It was discovered that removalof the post-SiH₄ soak purging step processing at low pressures, did notimpact the thickness of the SiN layer formed per cycle. Specifically,the low pressure and efficient pumping effectively interrupted the SiH₄soaking step.

It was also discovered that the thickness of the SiN material depositedper cycle was improved with treatment by the N₂/Hr plasma relative tothe NH₃/N₂ plasma. Specifically, treatment with the N₂/Ar plasmaresulted in a thickness of material deposited per cycle of about 3-5 Å,whereas treatment with the N₂/NH₃ plasma resulted in a thickness ofmaterial deposited per cycle of about 2-5 Å.

FIGS. 25A and 25B show electron micrographs of densely-packedtopographic features bearing SiN CVD films formed at wafer temperaturesof 350° C., utilizing N₂+NH₃ and N₂+Ar plasma treatment, respectively.Comparison of FIG. 25A with FIG. 25B reveals the presence of Ar in thetreatment step to have increased N₂ dissociation, and improved filmmorphology and step coverage.

FIG. 25C shows an electron micrograph of densely-packed topographicfeatures bearing a SiN CVD film formed from N₂+Ar plasma at a higherwafer temperature of 430° C. Comparison of FIG. 25C with FIG. 25Breveals the increased temperature to have improved step coverage of theresulting film.

FIG. 25D shows an electron micrograph of less dense topography bearing aCVD SiN film formed from N₂+Ar plasma at the 430° C. wafer temperature.Comparison of FIG. 25D with FIG. 25C reveals that the Pattern LoadingEffect (PLE) is also improved by this deposition regime.

Without being bound by any particular theory, the N₂ treatment reducesthe hydrogen content in the film leading to the formation of strainedSi—N bonds. By introducing additional steps (such as purge and/or pump)after deposition, the effect of the N₂ treatment is enhanced becausethere are no more deposition gases in the chamber. When residual SiH₄and NH₃ are still in the chamber, the deposition continues during thetreatment as well, and the treatment is able to penetrate through thematerial already deposited during the intentional deposition step.

FIGS. 26A-B are enlarged cross-sectional micrographs showing the upperportion of a raised feature bearing a SiN layer formed by a SiH₄soaking, followed by treatment with N₂/Ar and N₂/NH₃ plasmas,respectively. Comparison of FIGS. 26A-B shows that the nitride layertreated with the N₂/NH₃ plasma exhibits a columnar, grainy filmmorphology as compared with the film exposed to the Ar/N₂ plasma.

The character of the resulting film may depend upon the SiH₄ soakdeposition step, as well as upon the subsequent treatment with anitrogen-containing plasma. For example, the quality of morphology ofthe resulting deposited film may be influenced by the exposure doseduring the SiH₄ soak step. For purposes of the following discussion, theexposure dose is defined by Equation (I) below:D=T×PP; where D=exposure dose; T=time of exposure; and PP=partialpressure of SiH₄.  (I)

The thickness of the saturated film per dep/treat cycle is dependentupon the incoming flux of SiH₄ reaching the surface, and the rate ofdesorption of SiH₄ from that surface. The incoming SiH₄ flux isdependent upon the exposure dose, and the rate of SiH₄ desorption isdependent upon the temperature. Accordingly, FIG. 27 plots the rate ofdeposition of material versus exposure dose. FIG. 27 shows that thisrate of deposition will decline with increasing temperature.

FIG. 28A plots deposition rate versus exposure dose. FIG. 28B shows across-sectional micrograph showing a feature bearing a layer depositedafter a SiH₄ exposure dose of 500 mT*s. FIG. 28B shows the step coverageperformance of the silicon nitride film deposited with the processconditions corresponding to the fourth data point of FIG. 28A.

FIGS. 29A-H are cross-sectional electron micrographs showing themorphology of films deposited utilizing a SiH₄ soak deposition stepfollowed by treatment by exposure to a nitrogen-containing plasma underthe plurality of different conditions shown below in Table XI. TABLE XIRelative Film FIG. # Purge? Temp (° C.) Treat Plasma Morphology 29A No400 NH₃ Columnar 29B N₂ + Ar Homogenous 29C 500 NH₃ Columnar 29D N₂ + ArHomogenous 29E Yes 400 NH₃ Columnar 29F N₂ + Ar Homogenous 29G 500 NH₃Columnar 29H N₂ + Ar Homogenous

FIGS. 29A-H emphasize the effect of temperature and addition of Arduring treatment, upon step coverage and morphology of the resultingdeposited film. These figures indicate that higher temperature improvesboth step coverage and film morphology. In particular, step coveragefrom feature side wall-to-top (S/T), is improved from 30% at 400° C., to60% at 500° C. Film morphology is improved from columnar/grainy, todense and homogeneous films.

Without being bound by any theory, in comparing treatment by NH₃(typically diluted w/N₂) with treatment by N₂+Ar, the latter is morebeneficial because the addition of Ar increases plasma density byimproving the N₂ dissociation. This provides more N-radicals and ions toreact with the SiH₄ already present on the surface from the previousSiH₄ soak.

It has further been discovered that exposure to a plasma including Argongas can substantially enhance the rate of deposition of a film formed inaccordance with embodiments of the present invention.

For example, while the discussion so far has focused upon a processingregime wherein SiN is deposited from a plasma including both SiH₄ andNH₃, this is not required by the present invention. In accordance withalternative embodiments of the present invention, a ratio of NH₃: SiH₄can be zero (0) with material deposited in the absence of a plasma. Insuch an embodiment, an amorphous silicon layer (a-Si) is initiallydeposited from SiH₄ at low pressures. This amorphous silicon layer isthen subsequently treated with a plasma containing nitrogen, and alsopotentially Argon and Helium, in order to result in formation of theSiN.

The efficiency of the deposition process is in part limited by thesurface coverage of the first precursor. The silicon-source precursorhas to be chemisorbed on both the initial as well as the newly formedsurfaces with a 100% surface coverage. However, it is known thatnitrogen-containing precursors inhibit the adsorption of the silane(SiH₄) on the surface, which may lead to a decrease in the depositionrate with the number of cycles.

Maintaining a constant deposition rate throughout the deposition processis important to control film thickness. A substantially constantdeposition rate can be achieved using a surface activation process whichremoves the un-reacted precursors and enhances the chemisorption of SiH₄on the SiNx surface. The surface activation process may be realized byemploying an argon (Ar) cleaning step. The role of the Ar radicals is tosputter off the excess precursor adsorbed on the surface.

In accordance with embodiments of the present invention, Ar is eitherintroduced into the chamber for stabilization or after being stabilizedthrough the divert line. The Ar is radicalized using either a capacitiveplasma discharge inside the chamber, or using a Remote Plasma Unit(RPS). The plasma power, gas flow and cleaning time are parametersinfluencing the surface recovery.

FIG. 14 shows the difference in the plots film thickness versus numberof cycles for regimes including and not including a post-treatment Arplasma cleaning step. Without the Ar clean, the deposition ratedecreases 10 times over 120 cycles. When using the Ar clean, a constantdeposition rate of about 0.5 ML/cycle is achieved. A similarpost-treatment cleaning concept can also be used to form other types offilms, for example other barrier dielectric films.

FIG. 15 plots SiN film thickness versus a number of dep-soak(SiH₄)/treat (NH₃) cycles under the specific conditions listed below inTable VII: TABLE VII Cycle Step Step SiH₄ NH₃ NH₃ Process Parameter soakPurge stab plasma Purge Step time (sec) 20 10 10 10 10 Pressure (Torr)4.8 4.8 4.8 4.8 4.8 HF RF Power (W) 0 0 0 400 0 Heater temperature (C.)400 400 400 400 400 Lift Pos (steps) 450 450 450 450 450 SiH₄ Flow(sccm) 230 0 0 0 0 NH₃ Flow (sccm) 0 0 100 100 0 N₂ Flow (sccm) 20002000 2000 2000 2000

FIG. 15 indicates that the deposition rate decreases over time from 8Å/cycle to less than 1 Å/cycle. This decrease in deposition rate may bedue to the accumulation of NH₃ and NH₃-derived species on the surface ofthe film after each dep/treat cycle. Accordingly, a cleaning step may beemployed at the end of each cycle to regenerate and prepare the surfaceto adsorb SiH₄ in the deposition stage of the next cycle.

FIG. 16 plots thickness of SiN films deposited utilizing the cyclic depprocess regime described in Table VII, where the surface is exposed todifferent conditions between successive dep/treat cycles. FIG. 16 showsexposure to an Argon plasma to be the most effective inter-cyclecleaning approach.

FIG. 17 plots thickness of SiN films deposited utilizing the cyclicprocess regime described in Table VII, where the surface is exposed todifferent conditions between successive dep/treat cycles. FIG. 17 showsthat reduction in power applied to generate the Ar cleaning plasma,resulted in a further improvement in deposition rate.

V. Embodiments of Substrate Processing Chambers

An embodiment of a substrate processing chamber 80 that can be used fordepositing stressed materials is schematically illustrated in FIG. 30.While an exemplary chamber is used to illustrate the invention, otherchambers as would be apparent to one of ordinary skill in the art mayalso be used. Accordingly, the scope of the invention should not belimited to the exemplary embodiment of the chamber or other componentsprovided herein. Generally, the chamber 80 is a plasma enhanced chemicalvapor deposition (PE-CVD) chamber suitable for processing a substrate32, such as a silicon wafer. For example, a suitable chamber is aProducer® SE type chamber from Applied Materials, Santa Clara, Calif.The chamber 80 comprises enclosure walls 84, which include a ceiling 88,sidewalls 92, and a bottom wall 96, that enclose a process zone 100. Thechamber 80 may also comprise a liner (not shown) that lines at least aportion of the enclosure walls 84 about the process zone 100. Forprocessing a 300 mm silicon wafer, the chamber typically has a volume ofabout 20,000 to about 30,000 cm³, and more typically about 24,000 cm³.

During a process cycle, the substrate support 104 is lowered and asubstrate 32 is passed through an inlet port 110 and placed on thesupport 104 by a substrate transport 106, such as a robot arm. Thesubstrate support 104 can be moved between a lower position for loadingand unloading, and an adjustable upper position for processing of thesubstrate 32. The substrate support 104 can include an enclosedelectrode 105 to generate a plasma from process gas introduced into thechamber 80. The substrate support 104 can be heated by heater 107, whichcan be an electrically resistive heating element (as shown), a heatinglamp (not shown), or the plasma itself. The substrate support 104typically comprises a ceramic structure which has a receiving surface toreceive the substrate 32, and which protects the electrode 105 andheater 107 from the chamber environment. In use, a radio frequency (RF)voltage is applied to the electrode 105 and a direct current (DC)voltage is applied to the heater 107. The electrode 105 in the substratesupport 104 can also be used to electrostatically clamp the substrate 32to the support 104. The substrate support 104 may also comprise one ormore rings (not shown) that at least partially surround a periphery ofthe substrate 32 on the support 104.

After a substrate 32 is loaded onto the support 104, the support 104 israised to a processing position that is closer to the gas distributor108 to provide a desired spacing gap distance, d_(s), therebetween. Thespacing distance can be from about 2 mm to about 12 mm. The gasdistributor 108 is located above the process zone 100 for dispersing aprocess gas uniformly across the substrate 32. The gas distributor 108can separately deliver two independent streams of first and secondprocess gas to the process zone 100 without mixing the gas streams priorto their introduction into the process zone 100, or can premix theprocess gas before providing the premixed process gas to the processzone 100. The gas distributor 108 comprises a faceplate 111 having holes112 that allow the passage of process gas therethrough. The faceplate111 is typically made of metal to allow the application of a voltage orpotential thereto, and thereby serve as electrode in the chamber 80. Asuitable faceplate 111 can be made of aluminum with an anodized coating.The substrate processing chamber 80 also comprises first and second gassupplies 124 a, b to deliver the first and second process gas to the gasdistributor 108, the gas supplies 124 a, b each comprising a gas source128 a, b, one or more gas conduits 132 a, b, and one or more gas valves144 a, b. For example, in one version, the first gas supply 124 acomprises a first gas conduit 132 a and a first gas valve 144 a todeliver a first process gas from the gas source 128 a to a first inlet110 a of the gas distributor 108, and the second gas supply 124 bcomprises a second gas conduit 132 b and a second gas valve 144 b todeliver a second process gas from the second gas source 128 b to asecond inlet 110 b of the gas distributor 108.

The process gas can be energized by coupling electromagnetic energy, forexample, high frequency voltage energy to the process gas to form aplasma from the process gas. To energize the first process gas, avoltage is applied between (i) the electrode 105 in the support 104, and(ii) a second electrode 109 which may be the gas distributor 108,ceiling 88 or chamber sidewall 92. The voltage applied across the pairof electrodes 105, 109 capacitively couples energy to the process gas inthe process zone 100. Typically, the voltage applied to the electrode105, 109 is at a radio frequency. Generally, radio frequencies cover therange of from about 3 kHz to about 300 GHz. For the purposes of thepresent application, low radio frequencies are those which are less thanabout 1 MHz, and more preferably from about 100 KHz to 1 MHz, such asfor example a frequency of about 300 KHz. Also for the purposes of thepresent application, high radio frequencies are those from about 3 MHzto about 60 MHz, and more preferably about 13.56 MHz. The selected radiofrequency voltage is applied to the first electrode 105 at a power levelof from about 10 W to about 1000 W, and the second electrode 109 istypically grounded. However, the particular radio frequency range thatis used, and the power level of the applied voltage, depend upon thetype of stressed material to be deposited.

The chamber 80 also comprises a gas exhaust 182 to remove spent processgas and byproducts from the chamber 80 and maintain a predeterminedpressure of process gas in the process zone 100. In one version, the gasexhaust 182 includes a pumping channel 184 that receives spent processgas from the process zone 100, an exhaust port 185, a throttle valve 186and one or more exhaust pumps 188 to control the pressure of process gasin the chamber 80. The exhaust pumps 188 may include one or more of aturbo-molecular pump, cryogenic pump, roughing pump, andcombination-function pumps that have more than one function. The chamber80 may also comprise an inlet port or tube (not shown) through thebottom wall 96 of the chamber 80 to deliver a purging gas into thechamber 80. The purging gas typically flows upward from the inlet portpast the substrate support 104 and to an annular pumping channel. Thepurging gas is used to protect surfaces of the substrate support 104 andother chamber components from undesired deposition during theprocessing. The purging gas may also be used to affect the flow ofprocess gas in a desirable manner.

A controller 196 is also provided to control the activities andoperating parameters of the chamber 80. The controller 196 may comprise,for example, a processor and memory. The processor executes chambercontrol software, such as a computer program stored in the memory. Thememory may be a hard disk drive, read-only memory, flash memory or othertypes of memory. The controller 196 may also comprise other components,such as a floppy disk drive and a card rack. The card rack may contain asingle-board computer, analog and digital input/output boards, interfaceboards and stepper motor controller boards. The chamber control softwareincludes sets of instructions that dictate the timing, mixture of gases,chamber pressure, chamber temperature, microwave power levels, highfrequency power levels, support position, and other parameters of aparticular process.

The chamber 80 also comprises a power supply 198 to deliver power tovarious chamber components such as, for example, the first electrode 105in the substrate support 104 and the second electrode 109 in thechamber. To deliver power to the chamber electrodes 105, 109, the powersupply 198 comprises a radio frequency voltage source that provides avoltage having the selected radio frequencies and the desired selectablepower levels. The power supply 198 can include a single radio frequencyvoltage source, or multiple voltage sources that provide both high andlow radio frequencies. The power supply 198 and also include an RFmatching circuit. The power supply 198 can further comprise anelectrostatic charging source to provide an electrostatic charge to anelectrode often electrostatic chuck in the substrate support 104. When aheater 107 is used within the substrate support 104, the power supply198 also includes a heater power source that provides an appropriatecontrollable voltage to the heater 107. When a DC bias is to be appliedto the gas distributor 108 or the substrate support 104, the powersupply 198 also includes a DC bias voltage source that is connected to aconducting metal portion of the faceplate 111 of the gas distributor108. The power supply 198 can also include the source of power for otherchamber components, for example, motors and robots of the chamber.

The substrate processing chamber 80 also comprises a temperature sensor(not shown) such as a thermocouple or an interferometer to detect thetemperature of surfaces, such as component surfaces or substratesurfaces, within the chamber 80. The temperature sensor is capable ofrelaying its data to the chamber controller 196 which can then use thetemperature data to control temperature of the processing chamber 80,for example, by controlling the resistive heating element in thesubstrate support 104.

The embodiment of the chamber described above in connection with FIG. 30is typically configured to perform processing at pressures of about 1Torr and above. As shown and described above, however, in order todeposit highly conformal films with reasonably high throughput, it maybe advantageous to perform processing in a substantially lower pressureregime.

Accordingly, FIG. 31 shows a simplified cross-sectional view comparingthe processing chambers of the Applied Materials' Producer® SE chamberconfigured to operate at higher (≧1 Torr) pressures, and an alternativechamber embodiment configured to operate at lower (˜20-150 mTorr)pressures. FIG. 32 shows a perspective view of the modified chamber.

Lower pressure chamber 3100 differs from higher pressure chamber 3102 inthe following respects. First the low pressure chamber 3100 has beenmodified to increase the volume 3102 under the heater 3104 in order toimprove turbo pumping symmetry and efficiency. This allows the modifiedchamber to accommodate a higher power pump (not shown) with an adaptertube 3103 and a turbo throttle valve (not shown) and adding a new turboadapter tube parts to accommodate these parts. The chamber wasredesigned to create a low pressure pump port 3106 located atapproximately the height of the wafer pedestal 3108. This in turninvolved deepening the chamber body profile by about 2″, which in turninvolved extending lift pin rods 3110 and heater adapter block 3112 byabout the same distance.

In the region overlying the pedestal, isolators 3114 having a measuredthickness were employed, and spacers (not shown) were used to raise lidcomponents as necessary.

Different types of stressed materials can be deposited in accordancewith embodiments of the present invention. One type of stressed materialthat is commonly deposited comprises silicon nitride. By silicon nitrideit is meant a material having silicon-nitrogen (Si—N) bonds, includingmaterials such as silicon oxy-nitride, silicon-oxygen-hydrogen-nitrogen,and other stoichiometric or non-stoichiometric combinations of silicon,nitrogen, oxygen, hydrogen and even carbon. Exemplary methods to depositsilicon nitride stressed material are described herein to illustrate theinvention; however, it should be understood that these methods can alsobe used to deposit other types of materials, including stressed siliconoxide, stressed dielectric layers, and others. Thus, the scope of thepresent invention should not be limited to the illustrative stressedsilicon nitride embodiment described herein.

Although exemplary embodiments of the present invention are shown anddescribed, those of ordinary skill in the art may devise otherembodiments which incorporate the present invention, and which are alsowithin the scope of the present invention. For example, other radiationtreatments, such as infrared radiation or selected wavelengths ofvisible light may also be used to treat the deposited film. Also, acombination of different radiation exposures can also be used.Furthermore, the terms below, above, bottom, top, up, down, first andsecond and other relative or positional terms are shown with respect tothe exemplary embodiments in the FIGS. and are interchangeable.Therefore, the appended claims should not be limited to the descriptionsof the preferred versions, materials, or spatial arrangements describedherein to illustrate the invention.

1. A method of forming silicon nitride, the method comprising: disposinga substrate including a surface in a processing chamber; and depositingsilicon nitride on the surface by exposing the surface in the processingchamber to a silicon-containing precursor in a hydrogen-containingplasma.
 2. The method of claim 1 wherein the hydrogen-containing plasmais formed in a gas mixture including molecular hydrogen.
 3. The methodof claim 1 wherein the hydrogen-containing plasma comprises at least oneprecursor selected from a germanium-containing precursor and acarbon-containing precursor.
 4. The method of claim 2 wherein the gasmixture further comprises Argon.
 5. The method of claim 4 wherein thegas mixture further comprises at least one component selected frommolecular nitrogen, Xenon, and Krypton.
 6. The method of claim 1 whereinthe silicon nitride exhibits a compressive stress of 2.8 GPa or higher.7. A method of forming silicon nitride, the method comprising: disposinga substrate including a surface in a processing chamber; exposing thesurface to a plasma in the absence of deposition conditions to removecontamination; and depositing silicon nitride layer over theplasma-treated surface in the presence of a hydrogen gas flow.
 8. Themethod of claim 7 wherein the plasma is formed from an ambientcomprising at least one component selected from ammonia, oxygen, andN₂O.
 9. The method of claim 7 wherein the silicon nitride is depositedin the presence of the plasma.
 10. The method of claim 7 furthercomprising forming a buffer layer over the treated surface prior todeposition of the silicon nitride.
 11. The method of claim 10 whereinthe silicon nitride is deposited as a film stack comprising aninitiation layer formed in the absence of the hydrogen gas flow,followed by a high compressive stress layer formed in the presence ofthe hydrogen gas flow.
 12. The method of claim 7 wherein the siliconnitride exhibits a compressive stress of 2.8 GPa or higher.
 13. A methodof forming silicon nitride, the method comprising: disposing a substrateincluding a surface in a processing chamber; forming a buffer layer overthe surface; and depositing silicon nitride over the buffer layer in thepresence of a hydrogen gas flow.
 14. The method of claim 13 whereinforming the buffer layer comprises depositing an oxide layer.
 15. Themethod of claim 14 wherein the silicon nitride is deposited as a filmstack comprising an initiation layer formed in the absence of thehydrogen gas flow, followed by a high compressive stress layer formed inthe presence of the hydrogen gas flow.
 16. The method of claim 13wherein the silicon nitride exhibits a compressive stress of 2.8 GPa orhigher.
 17. A method of preventing defects in a device, the methodcomprising: depositing a silicon nitride initiation layer on a surfacein the absence of a flow of hydrogen gas; depositing a high compressivestress silicon nitride layer on the silicon nitride initiation layer ina presence of a flow of hydrogen gas; and etching back the highcompressive stress silicon nitride layer with a diluted NF₃ plasma. 18.The method of claim 17 wherein the etch back process is performed in thesame chamber as the high compressive stress deposition process.
 19. Themethod of claim 17 wherein the etch back process is modulated so that ahigher etch rate occurs on isolated areas.
 20. The method of claim 17wherein the high compressive stress silicon nitride layer exhibits acompressive stress of 2.8 GPa or higher.